1. Field
The present invention relates generally to a filter for removing particulate matter from an exhaust gas, and more particularly to a monolithic particulate filter.
2. Description of Related Art
An exhaust system for an engine is often required to reduce particulate pollutants, and therefore typically includes a particulate filter. One type of particulate filter is useful for trapping diesel exhaust particulate, and, therefore is commonly referred to as a Diesel Particulate Filter (DPF). A typical DPF is constructed from a block of ceramic material, such as cordierite (Magnesium Aluminum Silicate) or silicon carbide. Sometimes the DPF is metallic. The cordierite block may be extruded with parallel channels, which are used to direct an exhaust gas through the filter from an inlet port to an outlet port. Generally, the DPF is constructed to function using a wall-flow process. Sometimes the DPF is constructed to take advantage of a donut-shaped geometry. In either case, the exhaust gas is forced to go through a wall where the filtration of the particulates takes place. However, known filters have been found to have undesirable effects related to mechanical strength, filter backpressure (becoming especially as the filter becomes loaded with particulates), regeneration efficiency and soot trapping & ash storage efficiencies.
The most common DFP design incorporates a wall flow process. With the wall-flow process, half of the parallel channels are blocked at the inlet side and the other half are blocked at the outlet side, forcing exhaust gas to at least once pass through the solid but porous filter walls as it travels from the exhaust gas source through the filter and, ultimately, to the atmosphere. In this way, gas enters the inlet channels at the inlet side, and because the channels are blocked, is forced through a porous wall into an adjacent outlet channel. The outlet channel then directs the filtered exhaust gas to an outlet port for transition into the atmosphere. Since all (or at least nearly all) the exhaust gas must pass through at least one porous wall, the wall flow process may more effectively remove soot. However, in this process, trapped soot rapidly cakes on the surface of the channel wall, thus progressively blocking gas flow paths through the porous walls and contributing to (typically quick) a rise in backpressure. Also, since the gases are being forced though the walls, even newly constructed or freshly regenerated wall-flow filters may be characterized by unacceptably high backpressures.
In general, DPF designs to date suffer from such cake filtration effects arising from soot build-up on the wall surfaces. As soot collects on top of the surface of the porous wall, the effective diameter of the channel is reduced, leading to a sharp rise in backpressure. This soot must be occasionally burned off to clear the filter and regenerate the filtering effects of the DPF. This regeneration may be done in response to the detection of increased backpressure, in response to the detection of an excess level of particulate matter in the exhausted gases, or simply as a routine precaution. It will be appreciated that several methods for regeneration are well known. For example, the DPF may be heated during normal operation to a temperature sufficient to regenerate the filter. Alternately, the fuel system may inject fuel into the filter from time to time, thereby increasing the filter's temperature to facilitate burn off. Still alternately, the filter may be manually removed from the exhaust system and heated burn off accumulated soot.
Although regeneration is an important aspect of DPF design and use, the threat of an uncontrolled regeneration limits the practicability of automated regeneration processes. For example, an uncontrolled regeneration may result when, during normal regeneration or even during normal operation, the natural heat dissipation processes are interrupted. In one specific example, a DPF enters a regeneration cycle with the engine operating at normal highway cruising speeds. In this use, the DPF may reach a temperature of up to 700 to 900 degrees Celsius while regenerating. However, if the car were to suddenly stop, such as due to stop-and-go traffic, the engine speed would dramatically drop, and along with it the flow of exhaust gas through the DPF. Since exhaust gas flow is at least partially responsible for moving excess heat out of the DPF, such an event may trigger the temperature of the DPF to rise dramatically. In some cases, the temperate of the DPF may reach 1200 or 1300 degrees Celsius, or even higher.
In the presence of the very high temperatures observed in uncontrolled soot regenerations, some refractory ceramic materials exhibit undesirable reactions such as phase transitions or the formation of phases/eutectics in the presence of impurities. These impurities may be in the material itself, or extracted from the exhaust gases (such as ash-content in exhaust particulate matter). The reactions may cause a decrease in strength, melting point, or generate undesirable byproducts, resulting in physical weakening or chemical degradation of the DPF. In some cases, such as reactions in the DPF may also lead to sintering of the catalyst and washcoat, thereby reducing their efficiency. In one example, free silica in glass fibers can “flow” or creep at high temperatures leading to a substantial decrease in the strength of the filter body. Additionally, at temperatures above 1300 C, silica can also convert to crystalline form of cristobalite that may have negative health effects. Under high thermal gradients experienced during such regeneration events, the substrate may experience severe thermally induced stresses, leading to cracks and faults.
With the undesirable backpressure and regeneration characteristics of cordierite and other similar refractory ceramics, other materials and processes have been tried in the filtration of particulates from exhaust stream. For example, silicon carbide has exhibited promising material characteristics, but is extremely heavy, expensive, and filters are typically constructed of several blocks joined together with an adhesive, such as a cement or glue. These joined blocks are subject to breakage, are difficult to form into a precise honeycomb arrangement and often suffer from increased back pressure from fluid flow discontinuities inherent in the use of adhesive at the block-block interfaces. Accordingly, a silicon carbide DPF is typically too expensive, too heavy, and too difficult to manufacture for mass production use. Typically in the automotive industry, the ratio of liters of substrate to the engine displacement ranges from about 1 to about 2. This means that for a 6 liter engine, a full DPF system would require about 6-12 liters of honeycomb substrate (given the existing state of filtration engineering and ash storage capabilities). This would make the exhaust system prohibitively heavy, and would contribute to vehicle instability, necessitate under-body redesign and balancing, and would be accompanied by an inherent fuel penalty.
In another alternative, the DPF is formed from a block of ceramic that includes ACM mullite whiskers. Such mullite whiskers are typically single crystal mullite and have a needle morphology. The addition of these mullite needles improves refractory characteristics, and may also increase block porosity. For example, when used in a ceramic block, the porosity of the block may be increased to about 60%. However a filter constructed using ACM needles still exhibits an undesirably high backpressure, as well as suffering from relatively low wall strength. The process for creating these whisker-based ceramics is extremely expensive, often requiring expensive gases that are potentially dangerous to human health and industrial equipment. In such systems, it also becomes necessary to plug the substrates after the initial firing of the ceramic pre-cursor material, increasing in total cost of the wall flow DPF substrate.
Another type of filter is the fiber-wound or donut-shape substrate that includes metallic or ceramic fibers in a donut shape substrate. Such designs were created for HEPA air-filtration and are now being applied to exhaust remediation. Such designs provide low surface area for soot regeneration, are typically mechanically weak and contribute to rapid back-pressure increases with soot trapping.
Accordingly, there is a need for a particulate filter, and, in particular, a DPF, that efficiently captures soot, does not contribute excessively to backpressure, and can safely survive the rigors of uncontrolled regeneration. The present invention addresses this need.